Cranes which include a boom capable of being raised and lowered and a derricking mechanism for raising and lowering the boom are conventionally known. Patent Literature 1 mentioned below discloses an example of such cranes.
In the crane disclosed in Patent Literature 1, the boom has a proximal end mounted on a front portion of a machine body of the crane rotatably about an axis extending in a widthwise direction of the machine body. On the machine body, a live mast is disposed behind the boom and a high mast is disposed behind the live mast. Each of the live mast and the high mast has a proximal end mounted on the machine body rotatably about an axis extending in the widthwise direction of the machine body so as to be rotatable about the proximal end.
A top link is disposed on a ventral surface of a distal end of the boom. A top sheave is disposed at the top link, and a hook for hanging a hoisting load is suspended from the top sheave via a hook rope. A pendant link is secured to a back surface of the distal end of the boom. A rear end of the pendant link and a distal end of the live mast are connected via a pendant corresponding to a guy line.
A boom rope is wound around a sheave disposed at a distal end of the high mast and a sheave disposed at the distal end of the live mast, and is wound in and out by a winch disposed on the machine body. The winch winds in the boom rope to rotate the live mast rearward so that the distal end of the live mast approaches the distal end of the high mast. Consequently, the pendant is drawn rearward to draw the distal end of the boom rearward so that the boom is positioned vertically.
In this crane, owing to the pendant link projecting rearward from the distal end of the boom, the connection position of the pendant is spaced behind the distal end of the boom. Consequently, the acting position of a compressive force lies at a position close to an axis of the boom, the compressive force acting on the boom as the resultant of a tensile force of the pendant and a hoisting load. This allows the boom to withstand a great compressive force to thereby improve the hoisting capacity of the crane.
It is common to use a boom having a structure that allows the length of the boom to be changed, and change the length of the boom to an appropriate length depending on the contents of hoisting wok, the conditions of work site, or the like. In the above-described crane of Patent Literature 1, the hoisting capacity is improved by providing the pendant link; however, there is a possibility that the hoisting capacity cannot be prevented from decreasing when the boom is made shorter. Further, when the boom is made longer, another problem is liable to occur that the tensile force required to raise the boom being in the horizontal position increases.
Specifically, when the boom is made shorter, the angle between the extension direction of the pendant and the acting direction of a hoisting load increases. Consequently, the acting position of a compressive force is spaced in front of the axis of the boom, which results in a greater bending moment. Consequently, the hoisting capacity cannot be prevented from being reduced in order to avoid deflection of the boom. On the other hand, when the boom is made longer, the angle between the pendant and the axis of the boom decreases in a substantially horizontal position of the boom. This makes the vertical component of a tensile force of the pendant small. As a result, a greater tensile force of the pendant is required to raise the boom.